Interprocess
Communication IPC

Since processes run in their own memory space, they can not
communicate with each other without passing through the
kernel.

One way to communicate is to use the standard input and
output. Instead of connection the terminals keyboard and
console, an other process can be used.

One of the easiest way is sending signals to processes.
Processes can send signals via the kernel but they can also
have signal handles to have custom code to act when a signal
arrives. The signals can not contain data, they are used to
send events. The event is identified by the signal number that
is defined in /usr/include/bits/signum.h. Some commonly
used signals are: